Field of Invention
The present invention relates to an optical sheet for a liquid crystal display device, and a backlight unit for the liquid crystal display device.
Description of the Related Art
Backlight systems, which illuminate a liquid crystal layer from behind, are in widespread use as a transmissive liquid crystal display device, and the transmissive liquid crystal display device is equipped with an edge-lit (side-lit) backlight unit, a direct-lit backlight unit or the like on the back face side of the liquid crystal layer. As shown in FIG. 21, such an edge-lit backlight unit 101 is generally equipped with LED light sources 102, a light-guiding sheet 103 in the form of a rectangular plate disposed with an end face thereof being aligned with the LED light sources 102, and optical sheets 104 superimposed on the front face side of the light-guiding sheet 103. The optical sheets 104 have optical functions of, for example, diffusing and refracting the transmitted rays of light, and sheets used as the optical sheets 104 include a light diffusion sheet 105 which is disposed on the front face side of the light-guiding sheet 103 and primarily has a function of diffusing rays of light, a prism sheet 106 which is disposed on the front face side of the light diffusion sheet 105 and has a function refracting rays of light toward a normal direction, and the like (see, Japanese Unexamined Patent Application, Publication No. 2005-77448).
With regard to the functions of the backlight unit 101, in brief; first, rays of light having been emitted by the LED light sources 102 and entering the optical waveguide sheet 103 are reflected on reflection dots or a reflection sheet (not shown in FIG. 21) on the back face of the light-guiding sheet 103 as well as on each lateral face, and exit from the front face of the light-guiding sheet 103. The rays of light having exited from the front face of the light-guiding 103 enter the light diffusion sheet 105, are diffused and exit from its front face. The rays of light having exited from the front face of the light-diffusion sheet 105 enter the prism sheet 106, refracted by protruding-streak prism portions formed on the front face of the prism sheet 106 toward the normal direction and emitted, whereby the entire liquid crystal layer on the front face side (not shown in FIG. 21) is illuminated with the rays of light. Although not shown in FIG. 21, sheets which may be used as the optical sheets 104 include: an upper-layer diffusion sheet which is disposed on the front face side of the prism sheet 106 and slightly diffuses the rays of light to inhibit the lack in uniformity of luminance caused by, for example, the shapes of protruding prism portions of the prism sheet 106; a micro lens sheet which has a refraction function toward the normal direction and a wide-angle light diffusion function; and the like.
In the backlight unit 101, the LED light sources 102 are disposed along one end face of the light-guiding sheet 103, and thus, the rays of light exiting from the front face of the LED light sources 103 have such a directivity that a large amount of the rays of light are inclined toward the light emission direction of the LED light sources 102. The backlight unit 101 improves the luminance in the front direction in such a manner that the rays of light having such a directivity are diffused by the light diffusion sheet 105, and directed upward by the prism sheet 106 to travel in the normal direction.